Soundproofing panel structure



April 30, 1963 B. G. WATTERS 3,087,566

SOUNDPROOFING PANEL STRUCTURE Filed Dec. 30, 1958 2 Sheets-Sheet 1 fly. 1

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ATTORNEY-5' A ril 30, 1963 B. e. WATTERS SOUNDPROOFING PANEL STRUCTURE Filed Dec. 50, 1958 2 Sheets-Sheet 2 Rmvpou Lvcmeur TMMSMISS/OIV Loss -x THRESHOLD TLP LR l I l 1 I l 1 B00 FREQUENCY (Cvcuis PER SICOND) AVERAGE Tunasnow TRANSMISSION Loss TL DB) IO PANEL WEIGHT Guns/F1 INVENTOR. 3.1L & Mama BYMWM ATTORNEYS United States Patent 3,087 ,566 SOUNDPROOFING PANEL STRUCTURE Bill G. Watters, Nahant, Mass, assignor to Bolt Beranek and Newman Inc., Cambridge, Mass., a corporation of Massachusetts Filed Dec. 30, 1958, Ser. No. 783,800 4 Claims. (Cl. 181-33) The present invention relates to soundproofing methods and apparatus, and, more particularly, to compositematerial wall panels and the like.

A previously irreconcilable problem has existed for some time in the design of wall panels and the like for ofiices, homes and other structures, in view of the fact that the architect demands structural rigidity in such panels, whereas the acoustician, for purposes of preventing transverse vibration of the wall at frequencies approaching the velocity of sound that would give rise to acoustic radiation, prefers limp, or relatively limp, panel constructions. It happens, moreover, that by insisting upon rigid structures, the architect generally designs a wall structure and the like in such a way that it inherently permits such transverse propagation of vibrational waves and the resulting radiation of sound. Resort has, therefore, had to be had to the costly and supplemental operation of applying external acoustically absorptive materials to the surfaces of the panels and the like.

An object of the present invention, however, is to pro vide a new and improved method of and apparatus for soundproofing that shall satisfy the requirements of both the architect and the acoustician without the necessity for supplemental soundproofing operations or structures.

A further object is to provide a novel construction of substantial structural rigidity that, nontheless, is imbued with substantially the same acoustic performance that would have been obtained had the structure actually been limp. This end is achieved by combining two or more materials having transmission-loss versus frequency characteristics that dilfer in a critical manner, as more fully hereinafter set forth.

Other and further objects will be explained hereinafter, and will be more particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawing,

FIG. 1 of which is a fragmentary longitudinal section of a panel embodying the present invention;

FIG. 2 is a similar view of a preferred modification; and

FIGS. 3 and 4 are graphs illustrating the performance of the same.

It has been found that building materials have a particular type of characteristic curve relating the variation of what might be termed the random sound-incidence transmission loss property to the product of the frequency of the incident acoustic energy and the surface weight of the material. Fir, plywood, plateglass, aluminum, dense concrete, sandblasted steel, lead, and the other materials employed in partitions, walls and panels and the like, have all been found to possess such a characteristic curve, having the properties typically illustrated in curve W of FIG. 3. Curve W, which happens to represent the characteristic of inch plywood, comprises an initial substantially linearly rising portion I at the relatively low acoustic frequencies, a leveled-off substantially flat or plateau region II, and, at the higher frequencies, a further rising portion III.

It has been empirically determined, in connection with many tests relating to the silencing of speech sounds produced in normal background noise and the like, that there exists for different panel weights, a rather critical value of plateau transmission loss II in the transmission-loss versus frequency characteristic, that is required to provide satisfactory sound isolation. On the articulation index scale, it was found, for example, that most of the people tested were satisfied with the isolation produced by about a seven percent value of articulation index for ofiice soundproofing. The critical value for the transmission loss plateau region II, therefore, must correspond to this threshold seven-percent articulation index. Other thresholds, of course, apply for the soundproofing of other types of installations and backgrounds. In FIG. 4, therefore, the average threshold plateau transmission loss TL expressed in decibels, is plotted along the ordinate as a function of the panel weight G, plotted along the abscissa in units of pounds per square foot (weight of panel in pounds for each poriton having a surface area of one square foot), for this particular illustrative seven percent articulation index criterion. It will be observed, for example, that a panel of weight one pound per square foot will require a plateau region II, FIG. 3, having a threshold plateau transmission loss TL of about 23 decibels. A panel of two pounds per square foot, on the other hand, requires a TL of about 29' decibels; a panel of three pounds per square inch, a TL of about 33 decibels; and so on.

It has been determined that the approximate formula for the predetermined average threshold level TL of adequate quiet for such application is:

(1) TL =20 log G-l-K where G is the panel weight expressed in pounds per square foot and K is a constant lying within the range of from substantially 15 to substantially 35, with the preferred value for the seven percent articulation index being about 23.

Assume, for purposes of illustrating the invention, that an overall panel weight of about three pounds per square foot is desired, requiring, as before stated, a TL of about 33 decibels. This required plateau is plotted at the 33- decibel level as a horizontal dash-line in FIG. 3, labelled Threshold TI In accordance with the invention, one first selects a panel material that is too flimsy of itself to provide the structural rigidity demanded by an architect for wall panelling and the like, but that has the plateau region II of its transmission-loss versus frequency characteristic above the necessary threshold value TL Referring again to FIG. 3, a relatively limp sheet of steel, say 0.06 inch thick (16 gauge) has been selected, having the characteristics S the plateau region II of which lies well above the threshold value TL representing the before-mentioned seven percent articulation index that provides adequate acoustic transmission loss for satisfactory speech privacy in a typical Office. This thin steel panel S is thus sufficiently limp to provide adequate transmission loss, but it is unsatisfactory to the architect in view of its flimsy nature.

The invention further involves the selection of a rigidityproviding material, such as the previously discussed plywood W of FIG. 3, the plateau region II of which lies below the necessary threshold transmission loss value TL It has been found that if this rigidity-providing material W of lower-than-threshold plateau characteristic II is mechanically joined in juxtaposition with the relatively limp sheet S of higher-than-threshold plateau characteristic II, as by cement or other securing means 1, FIG. 1, a composite resultant panel characteristic will be produced having (1) the necessary plateau threshold transmission loss TL and (2) the necessary architectural structural rigidity. Both the architect and acoustician are thus provided with the panel features they each deem necessary and which, as before stated, have heretofore been considered inconsistent, and have thus required external acoustic tile or other surface treatment.

Curve R of FIG. 3 represents a theoretical resultant curve of the composite panel of FIG. 1, having the 0.06 inch steel sheet S laminated with the inch plywood layer W. In the determination of this theoretical resultant-an analysis gave rise to the following approximate relationships between the resultant panel weight G and density factor p, the panel loss tangent n (a dimensionless ratio proportioned to the internal dissipation loss resulting from bending), the total panel thickness H and the Youngs modulus E and E thickness H and H and density 1 and p of the respective laminated sheets:

(2) 30 log -10 log E+2 log a +48=20 log G where if 2 ii 3H1H E E-El HT ITE1 HT T 1 H ElH and n n PPl +P2 Actual tests have confirmed these theoretical predictions. With the preferred 0.06 inch steel sheet S, and the inch plywood panel W, laminated instwof parts at W and W FIG. 2, each about inch thick,'the experimentally obtained curve R FIG. 3, was obtained, corresponding well to the predicted curve R. Insofar as mechanical stiffness or rigidity of the composite panel is concerned, the composite panel was about ten times stiffer than the original thin steel sheet S, rendering the same entirely acceptable as an architectural structure, as well.

Further modifications will occur to those skilled in the art and all such areconsi-dered to fall within the spirit and scope of the invention as defined in the appended claims. The terms steel-like and wood-like employed in the claims are intended to embrace materials which are like steel or like wood with respect to the function performed by the designated elements in the panel of the invention. For example, materials other than steel may be employed as the relatively limp layer, and materials other than wood may be employed as the relatively rigid layer of the acoustic panel of the invention, provided the materials meet the criteria recited in the claims.

What is claimed is:

1. An acoustic panel for providing a predetermined average threshold level TL of adequate quiet for the space in which the panel is employed in accordance with the relationship: TL =2O log G+K, where G is the panel weight in pounds per square foot and K is a constant lying within the range of from substantially 15 to substantially 35, said panel comprising, in combination, a relatively limp layerof steel-like material having an acoustic transmission loss versus frequency characteristic curve with a plateau region above said level TL the mechanical rigidity of said layer being less than that required for structural support, and a relatively rigid layer of wood-like material having an acoustic transmission loss versus frequency characteristic curve with a plateau region below said level TL said rigid layer having substantially greater mechanical rigidity than said limp layer and supporting said limp layer, said layers being mechanically joined in juxtaposition, said panel having a total thickness H density factor p, and loss tangent and said layers having Youngs modulus E and E thickness H and H and density p and p respectively, all of which -conform substantially to the following relationships:

30 log p-IO log E+2 log 1j+48=20 log G where 2. The panel of claim 1, wherein the value of K is substantially 23.

3. The panel of claim 1, wherein the limp layer is steel of substantially 16 gauge and the rigid layer is wood of substantially inch thickness.

4. The panel of claim 1, wherein said rigid layer is formed in two parts on opposite sides of said limp layer.

References Cited in the file of this patent 'UNITED STATES PATENTS OTHER REFERENCES N. R. French and J. C. Steinberg, Factors Governing the intelligibility of Speech Sounds, The Journal of the Acoustical Society of America, vol. 19, No. 1, January 1947, pages -119.

Cyril M. Harris: Handbook of Noise Control (Mc- Graw-Hill Book Company, Inc., New York, 1957), pages 20-1 through 20-15, 20-18, 20-19, 20-26 and 20-27. 

1. AN ACOUSTIC PANEL FOR PROVIDING A PREDETERMINED AVERAGE THRESHOLD LEVEL TLP OF ADEQUATE QUIET FOR THE SPACE IN WHICH THE PANEL IS EMPLOYED IN ACCORDANCE WITH THE RELATIONSHIP: TLP=20 LOG G+K, WHERE G IS THE PANEL WEIGHT IN POUNDS PER SQUARE FOOT AND K IS A CONSTANT LYING WITHIN THE RANGE OF FROM SUBSTANTIALLY 15 TO SUBSTANTIALLY 35, SAID PANEL COMPRISING, IN COMBINATION, A RELATIVELY LIMP LAYER OF STEEL-LIKE MATERIAL HAVING AN ACOUSTIC TRANSMISSION LOSS VERSUS FREQUENCY CHARACTERISTIC CURVE WITH A PLATEAU REGION ABOVE SAID LEVEL TLP, THE MECHANICAL RIGIDITY OF SAID LAYER BEING LESS THAN THAT REQUIRED FOR STRUCTURAL SUPPORT, AND A RELATIVELY RIGID LAYER OF WOOD-LIKE MATERIAL HAVING AN ACOUSTIC TRANSMISSION LOSS VERSUS FREQUENCY CHARACTERISTIC CURVE WITH A PLATEAU REGION BELOW SAID LEVEL TLP, SAID RIGID LAYER HAVING SUBSTANTIALLY GREATER MECHANICAL RIGIDITY THAN SAID LIMP LAYER AND SUPPORTING SAID LIMP LAYER, SAID LAYERS BEING MECHANICALLY JOINED IN JUXTAPOSITION, SAID PANEL HAVING A TOTAL THICKNESS HT, DENSITY FACTOR P, AND LOSS TANGENT N, AND SAID LAYERS HAVING YOUNG''S MODULUS E1 AND E2, THICKNESS H1 AND H2 AND DENSITY P1 AND P2, RESPECTIVELY, ALL OF WHICH CONFORM SUBSTANTIALLY TO THE FOLLOWING RELATIONSHIPS: 